1. Field of the Invention
The invention relates to a method for improving the sensitivity and signal to noise ratio of piezo-electric transducers including a plurality of parallel combined sensors.
In the field of seismic prospection, reception devices are generally used often formed of a high number of seismic receivers disposed at a distance from each other and connected to a central recording laboratory. Each receiver includes an elementary sensor or several sensors interconnected in series and/or in parallel for increasing their voltage or load sensitivity respectively.
For sea applications, the receivers are disposed along a sealed sheath or seismic streamer towed immersed by a ship. The sensors generally used are of the piezoelectric type. They includes disks of relatively small dimensions made from a piezo-electric material and are each associated with a pair of electrodes. Those disks are contained in cases having one or more faces transparent to the acoustic wave.
The assembly of cases contained in a reception device is generally divided into a plurality of receivers, each of them including a plurality of cases spaced apart from each other.
The combination of the electric signals delivered by a relatively large number of sensors (20 to 30 for example) spaced apart at even intervals (of the order of 1 m for example) forms wavenumber sampling and, by processing, a part of the background noise is filtered.
It is a question of noise whose wavenumber is less than 500 cycles/km (seismic "firing" noise between 10 c/km and 20 c/km, acoustic noise etc).
For the noises whose wavenumbers are higher, particularly flow noises related to the advance of the ship, and to the eddies about the seismic streamer, filtering by processing is impossible because of the "aliasing" of the frequency spectrum (Shannon's theorem).
To reduce these noises, in particular those whose wavenumber is greater than 5000 c/km, which corresponds to reduction of a wavelength less than 0.2 m, the hydrodynamic drag of the streamer may be considered. But the most efficient solution consists in providing a continuous sensor whose length is greater than the greatest wavelength of the noises to be filtered, that is to say at least equal to 0.2 m in the example considered.
2. Description of the Prior Art
From U.S. Pat. No. 3,798,474, a sensor device is known formed of at least one sensitive element including a combination of a flexible ribbon made from piezoelectric material and two electrodes disposed on each end of the ribbon, and more generally from several sensitive elements interconnected in series for example, so as to increase the sensitivity of the whole.
The sensors of this type generally have a fairly low sensitivity, which leads to combining them.
As will be seen in greater detail in the description, the combination of elementary sensors each formed of a sensitive piezoelectric element associated with electrodes and connection thereof to an electric charge amplifier, increases their sensitivity and/or the ratio of the signal which they deliver to noises of different origins (S/N).
The case will be considered of combinations of several sensors in series which does not change the gain obtained by the assembly but substantially increases the S/N ratio.
The combination of several sensors in parallel increases the sensitivity, as will be seen, but leaves practically unchanged the S/N ratio obtained. The method of the invention increases both the sensitivity of a parallel combination of a plurality of elementary sensors and the signal to noise ratio.